The present invention relates to dental model articulators of a vertex type disclosed in U.S. Pat. Nos. 4,382,787; 4,449,930; 4,533,323; 4,548,581 and 4,734,033.
Dental articulators are widely used by dental laboratory technicians for holding and manipulating the dental castings of upper and lower sets of teeth while engaged in modeling dental prosthesis, such as a crown or a bridge for a missing tooth. Molds are first made of the patient""s upper and lower set of teeth from which castings are then made, either integral with plates of casting stone material for holding them or separately. In either case, the dental castings are supported by a pair of stone plates that serve as upper and lower jaws hinged together through two U-shaped brackets as disclosed in the aforesaid prior-art patents, which by this reference are hereby incorporated herein. Together, the brackets emulate condyles for articulation of the lower jaw relative to the upper jaw and are used by the technician for initially adjusting the bite between the sets of teeth on the two plates and for continually checking occlusion between the teeth castings and the prosthesis during the process of modeling the prosthesis.
The pair of U-shaped brackets are hinged together at the ends of their two parallel arms to permit movement of one bracket relative to the other bracket about their hinge axis. The base between parallel arms of each bracket is connected to the rear face of a stone plate through a ball of a ball-and-socket joint which constitutes a vertex for the bracket. The socket for each joint is affixed on the back of its stone plate by a separate mounting means, while support for the ball on its bracket base is provided by a stalk slightly offset from the center of the bracket base such that, once the ball joint connection is completed, the centers of the balls supported on the hinged brackets lie on a common plane perpendicular to the axis of the hinged brackets and centered between the hinged bracket arms, as more fully described and illustrated in each of the aforesaid prior-art patents. By offsetting the ball of each bracket on its base in that manner, both brackets can be cast from the same mold, each with a hinge pin extending outwardly extending from one arm and a slotted perforation to receive a hinge pin in the other arm. Upon orienting one bracket 180xc2x0 with respect to the other, the hinge pin of each may be snap-fit in the slotted perforation in an arm of the other bracket.
The sockets for the balls in the aforesaid prior-art patents are semispherical cavities which cannot hold the balls in their sockets while the dental castings are being initially aligned, much less during the modeling of the prosthesis. In order to provide a positive means for holding the balls in the cavities during initial alignment of the plates bearing the castings, curved fingers are provided that extend from the edges of the semispherical cavities over the balls. Each ball is snap-fit into its cavity past the fingers, but the clamping force of the fingers is insufficient to hold the balls in their position of initial alignment throughout the process of modeling the prosthesis. Consequently, it is necessary and sufficient to apply a fast setting glue, such as cynacrolate glue (commonly known as super glue) between the balls and the cavities after initial alignment. The importance of this glue in the prior-art patents referred to above is emphasized in the most recent U.S. Pat. No. 4,734,033. It shows a groove cast in the surfaces of each ball to facilitate flow of the glue into the cavity with the ball already in place. However, it is often necessary to realign the plates bearing the dental castings relative to each other during the process of modeling prosthesis. The use of glue to affix the ball in its socket then presents serious problems.
The problems arise from the need to remove the glue in order to free the bracket balls for realignment, which requires using a suitable solvent, such as acetate. This removal of glue is not only time consuming, but also dangerous because most solvents are highly volatile, flammable and toxic, particularly acetate, thus subjecting personnel in the dental laboratory to hazards that should be avoided. Moreover, the ball joints must stand in the solvent for a considerable time in order to fully dissolve the glue, which aggravates the risk of exposure not only to the toxicity of the solvent but also to the risk of fire or explosion in the laboratory. It would be preferable to use releasable mechanical means to affix the balls in the sockets, but previous attempts to accomplish that have not been successful.
One attempt has been to mechanically affix a ball in a socket using a set screw through one side of the socket to press the ball against the other side, thus clamping the ball between one point of contact set by the screw to force the ball in tangential contact with the socket wall on the opposite side of the ball. (See U.S. Pat. Nos. 4,797,097 and 5,007,829.) The frictional clamp created in that manner at two opposite points to the ball is not sufficient to prevent the ball from turning freely on an axis between those two points since tangential contact of the ball""s spherical surface with any surface of the clamp, flat or curved, is but a point of contact, unless a curved surface is provided for the clamp with a radius of curvature that matches the radius of the ball with precision. Unless that radius of curvature is so matched, the area of contact with the ball is but a virtual joint. Only extreme pressure of the ball against the clamping surface can change that by increasing the area of contact either by deforming the ball or the clamping surface to more precisely conform the curvature of one to that of the other over a significant area around the point directly opposite the set screw.
Another prior-art approach utilizes a clam-type socket clamp, as shown in U.S. Pat. No. 5,106,296. There a ball is set between opposing cavities that form the clam-type socket. A locking screw through extensions of the opposing cantilevered cavities forces the cavities against the ball. Regardless of how tight the screw is turned, the clamping force of the two opposing cavities against the ball will not produce sufficient frictional force to affix the ball because each of the opposing cavities only makes tangential contact with the ball, which can then turn on an axis between the contact points. (See also U.S. Pat. No. 4,200,981.) For any clamping that will affix the orientation of the ball in the socket, the radius of curvature for the cavities of the clam-type socket must be made to match the radius of the ball with precision, and the gap between the two opposite cavities clamping the ball must be sufficient to allow for increasing the pressure of the cavities against the ball without limit. Thus, a clam-type socket would require precision in forming the cavities.
An object of this invention is to obviate the need for glue as a means for affixing ball-and-socket joints in a dental model articulator by instead utilizing releasable mechanical means for adjusting the relative diameters between the socket and the ball in a ball-and-socket joint in order to releasably affix the ball in the socket by frictional force that surrounds the ball over a significant area without the need for precision matching the radius of curvature of the socket cavity with the radius of the balls.
In accordance with a first embodiment of the present invention, the clamping friction between a ball and truncated spherical socket of a ball-and-socket joint in a dental model articulator is provided by a concave C-clamp that serves as the socket for releasably affixing the ball in the socket.
The C-clamp comprises a concave band around the spherical surface of the ball with the radius of concave curvature of the band substantially equal to the radius of the ball. Two free ends of the concave band that forms the C-clamp are each provided with a separate flange and a gap between the flanges. One flange is perforated and internally threaded to receive the end of a bolt, and the other flange is perforated to pass the threaded end of the bolt to the one flange internally threaded. After snap-fitting a ball into the C-clamp while the gap between the flanges is free to expand, the bolt is turned in one direction into the one internally threaded flange to close the gap between the flanges and produce sufficient tension in the concave band in order to firmly grip the ball within the C-clamp. Allowing the concave curvature of the band to mold itself to the ball as the C-clamp socket is put under tension obviates the need for any precision in matching the radius of concave curvature of the band to the radius of the ball.
The C-clamp socket for each articulator bracket ball is preferably made as an integral part of mounting means for affixing the C-clamp to the rear face of a dental plate so that the ball for each articulator bracket may then be formed as an integral part of the articulator bracket, but the reverse is equally effective and therefore is to be regarded as being equally within the scope of the appended claims.
In an alternative embodiment of the ball-and-socket joint, a truncated spherical cavity having a diameter substantially equal to the diameter of the ball serves as the socket for the ball-and-socket joint. The truncated spherical cavity is formed as an integral part of the mounting means with its opening for receiving the ball away from the mounting plate thereof.. The diameter of that opening is, of course, less than the diameter of the ball by a predetermined amount that allows inserting the ball in the cavity, as further described below. The ball is formed with a stalk as an integral part of an articulator bracket to support the ball on the base of the bracket between its two arms. The bracket base and stalk are both perforated and threaded to receive a bolt.
The ball formed on the stalk over the base of the bracket is divided into four equal parts, each separated by equal gaps. Each gap between any two adjacent parts is perpendicular to gaps between those same two parts and two other parts, with the intersecting gap spaces between the four corners of all the parts centered on the axis of the threaded perforation through and stalk and base. The threaded bolt through the stalk is selected to have a thread diameter greater than the space between diametrically opposite corners of the parts, such that as the threads of the bolt advance between the four corners of the parts, the four parts spread equally, thus pressing them against the wall of the truncated cavity to affix the ball in the socket by frictional force.
Upon initially press-fitting the ball into the socket through the slightly smaller opening of the truncated spherical socket, the four gapped parts of the ball, held in gapped position by the stalk, are bent slightly toward each other at their connection with the stalk while being inserted into the truncated cavity, thus effectively reducing the diameter of the ball while it is being inserted through the cavity opening. Once the gapped ball is inserted into the cavity, the bolt is turned to force its end through the threaded supporting base and stalk into the gapped ball between the four parts, thus forcing apart the four parts and thereby affixing the ball in the cavity. In that manner, the gaps between the four ball parts serve two purposes: to allow each of the four parts having a radius of curvature substantially at the stalk into the gaps between them, i.e., to bend toward each other, for insertion of the ball into the cavity; and to receive a bolt between the parts to force them against the cavity wall.
The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.